1. Field of the Invention
The present invention relates to a semiconductor ring laser gyro using a semiconductor as a light source, and particularly to a semiconductor ring laser gyro capable of measuring rotational angular velocities about two axes with one semiconductor laser.
2. Description of the Related Arts
A gyroscope has been conventionally known as a means of measuring the rotational angular velocity of an object. Among others, a ring laser gyro, which utilizes the Sagnac effect, is adapted to precisely measure the rotational angular velocity and therefore is widely used, particularly in the aircraft and rocket industries. While an He—Ne gas laser is primarily used as s laser light source for the ring laser gyro described above, a semiconductor laser, which is advantageous in reduction of device size and power consumption, is recently used increasingly (for example, Japanese Patent Application Laid-Open No. 2001-50753, Japanese Patent Application Laid-Open No. 2003-139539, and Japanese Patent Application Laid-Open No. 2006-319104).
FIG. 6 is a top plan view of an example of a conventional semiconductor laser ring gyro. The semiconductor ring laser gyro includes a semiconductor laser 60 mounted on a silicon substrate, four mirrors 61 to 64, and interference light (beat light) pickup mirrors 65 and 66. The semiconductor laser 60 has its both ends provided with an antireflection coating and emits lights respectively from the both ends (refer to Japanese Patent Application Laid-Open No. 2006-319104). The lights emitted from the both ends of the semiconductor laser 60 are caused by the four mirrors 61 to 64 to travel in respective optical circuits in the right hand direction and the left hand direction, wherein light emitted from the semiconductor laser 60 enters an end thereof opposite to en end from which the light is emitted. The optical circuits function as a ring resonator, and a laser oscillation occurs at the both ends of the semiconductor laser 60. The four mirrors 61 to 64 are fabricated by anisotropic etching of a silicon substrate (silicon micromachining technique), and a metal coating or a dielectric multilayer coating is provided (refer to Japanese Patent Application Laid-Open No. 2003-139539, Paragraph 0037). At least one of the four mirrors 61 to 64 is a transmissive mirror adapted to introduce part of the light to the beat light pickup mirrors 65 and 66.
In the semiconductor ring laser gyro described above, when an object rotates about a rotation axis (sensitivity axis) defined by the normal line of the silicon substrate, an optical path difference is generated due to the Sagnac effect between the two paths of the lights traveling respectively in the right hand direction and the left hand directions, and a beat signal based on an oscillation frequency difference is detected. A rotational angular velocity Ω is calculated by a frequency Δf of the beat signal (refer to Patent document 3, Paragraph 0015) according to formula 1 shown below, where A is an area enclosed by the ring optical path, λ is an oscillation wavelength of the ring laser, and L is a length of the ring optical path:

To achieve camera image stabilization and construction equipment control, rotational angular velocities about two axes must be measured. The aforementioned semiconductor ring laser gyro built on a silicon substrate is capable of measuring rotational angular velocity about one axis only, and in order to measure rotational angular velocities about a plurality of axes, a number of semiconductor ring laser gyros corresponding to the number of axes are required. This pushes up cost of production and also causes a problem of installation space increase.